1. Field of the Invention
The present invention relates to an AC coupling circuit for coupling an AC output signal from an AC signal processing circuit to a circuit subsequent to the AC signal processing circuit.
2. Description of the Prior Art
In general, since the sensitivities of various sensors such as semiconductor sensors are very low, output signals from the sensors must be amplified to an appropriate level.
Sensor outputs are varied due to the variance of sensors or the like in manufacturing, and therefore the sensor outputs are hardly processed by DC signals. For this reason, the output signals from the sensors are amplified by a plurality of amplifying circuits. In this case, the output signals are coupled to a first-stage amplifier to reduce the influence of variance of outputs of DC signals (AC signal processing).
FIG. 5 shows the configuration of a conventional AC coupling circuit. A signal from a half bridge circuit comprising resistors R1 and R2 which serve as a sensor is input to a buffer A2 comprising a differential amplifier. At the same time, a reference potential is input from a half bridge circuit comprising resistors R3 and R4 to a buffer A1 comprising by a differential amplifier. The buffers A1 and A2 perform impedance conversion. Outputs from the buffers A1 and A2 are input to a first-stage amplifier A3 and differentially amplified by using a reference potential given by the resistors R5 and R6. Then the outputs are transmitted to a stage subsequent to the first-stage amplifier A3 through an AC coupling circuit 11. As the AC coupling circuit 11, a CR circuit comprising a capacitor C1 and a resistor R16 is generally used, as shown in FIG. 1.
In this case, due to the variance of DC signals of sensor outputs, the potential difference between the potential of a stage (that is, the first-stage amplifier A3) previous to the AC coupling circuit 11 and the reference potential of a stage subsequent to the AC coupling circuit 11 may be large. In addition, the previous stage output also has temperature characteristic due to the temperature characteristic of the sensor output. For this reason, an increase in the temperature raises the potential difference. More specifically, when the DC potential component in the sensor output (potential of the middle point between the resistor R1 and the resistor R2) is varied due to a variance in manufacturing or temperature characteristic, this variation causes the output potential of the first-stage amplifier A3 to vary with respect to the reference potential of the subsequent stage obtained by the resistor R11 and the resistor R12. That is, a potential difference is generated between the output potential of the first-stage amplifier A3 and the reference potential of the subsequent stage obtained by the resistor R11 and the resistor R12.
When no power supply voltage is applied, the a capacitor C1 of the AC coupling circuit 11 is not charged. When the power supply voltage is applied, the capacitor C1 is charged by the potential difference between the potential of the previous stage of the AC coupling circuit 11 and the potential of the subsequent stage of the AC coupling circuit 11. Thereafter, the potential of the subsequent stage reaches a stationary potential. FIG. 6 shows changes of the potential of the previous stage of the AC coupling circuit 11 and the potential of an output from the AC coupling circuit 11 after the power supply voltage is applied. In FIG. 6, a straight line P indicates the potential of a node P of the previous stage of the AC coupling circuit 11, and varies due to the variance of the sensor. A curve Q indicates the change in potential of an output (node Q) from the AC coupling circuit. As shown in FIG. 6, an output from the AC coupling circuit 11 varies until the charge corresponding to the potential difference is charged in a capacitor C1, and then is converged to a stationary potential expressed by R (reference potential obtained by the resistors R11 and R12).
As described above, when a potential difference across the AC coupling circuit is generated due to the variance of a sensor after a power supply voltage is applied, a predetermined elapsed time is required after applying the power supply voltage until the potential of the subsequent stage of the AC coupling circuit reaches a stationary potential. The elapsed time depends on the time constant of the AC coupling circuit, and the signal processing circuit cannot normally operate as an AC signal processing circuit during the elapsed time.
The present invention has been made to solve the above problem, and has its object to provide an AC coupling circuit which equalizes the potentials between nodes before and after the AC coupling circuit immediately after a power supply voltage is applied, thereby making the potential of the previous stage of the AC coupling circuit rapidly reach a stationary potential.
In a first aspect of the invention, an AC coupling circuit is provided for receiving an output signal from a signal processing circuit having a first differential amplifier which has an inverting input and a non-inverting input, and for transmitting the output signal to a stage subsequent to the signal processing circuit as an AC signal. The first differential amplifier receives an input signal with a predetermined potential and a signal with a reference potential to differentially amplify the signals. The AC coupling circuit comprises a CR circuit including a resistor and a capacitor, and a feedback circuit. The feedback circuit includes a second differential amplifier for detecting a potential difference between a node of a stage previous to the CR circuit and a node of a stage subsequent to the CR circuit with reference to the potential of the node of the subsequent stage to the CR circuit to amplify the potential difference. The feedback circuit feeds back the amplified potential difference to the non-inverting input of the first differential amplifier of the signal processing circuit.
According to the coupling circuit thus configured, the potentials of the previous stage and the subsequent stage to the AC coupling circuit can be made equal to each other immediately after a power supply voltage is applied, and the potential of the subsequent stage can be rapidly caused to reach a stationary potential. Therefore, even though the sensor has variance, the signal processing circuit connected to the AC coupling circuit can be rapidly and normally operated immediately after the power supply voltage is applied.
In the AC coupling circuit, the first differential amplifier may receive the input signal with a predetermined potential and the signal with a reference potential through a buffer circuit which has an output end connected to a resistor and performs impedance conversion. A product of a resistance of the resistor included in the CR circuit and a gain of the second differential amplifier may be equal to the resistance of the resistor connected to the output end of the buffer circuit.
Thus, the capacitance of a capacitor of the CR coupling circuit can be arbitrarily set, and the degree of freedom of selection of capacitors can be increased. Therefore the manufacturing cost of a signal processing circuit using this circuit can be reduced.
In the AC coupling circuit, the feedback circuit may further include a buffer circuit which has an output end connected to a resistor and performs impedance conversion. The potential difference amplified by the second differential amplifier may be fed back to the non-inverting input of the first differential amplifier of the signal processing circuit through the buffer circuit.
Thus, the capacitance of a capacitor in the CR circuit can be arbitrarily set without considering circuit constants, the degree of freedom of selection of capacitors can be increased.
In a second aspect of the invention, an AC coupling circuit is provided for receiving an output signal from a signal processing circuit having a first differential amplifier which has an inverting input and a non-inverting input, and for transmitting the output signal to a stage subsequent to the signal processing circuit as an AC signal. The first differential amplifier receives a input signal with a predetermined potential and a signal with a reference potential to differentially amplify the signals. The AC coupling circuit comprises a CR circuit including a resistor and a capacitor, and a feedback circuit. The feedback circuit includes a second differential amplifier for detecting a potential difference between a node of a stage previous to the CR circuit and an ode of a stage subsequent to the CR circuit with reference to the potential of the node of the previous stage to the CR circuit to amplify the potential difference. The feedback circuit feeds back the amplified potential difference to the non-inverting input of the first differential amplifier of the signal processing circuit. The second differential amplifier has a gain equal to a reciprocal value of a gain of the first differential amplifier.
According to the coupling circuit thus configured, the potentials of the previous stage and the subsequent stage to the AC coupling circuit can also be made equal to each other immediately after a power supply voltage is applied, and the potential of the subsequent stage can be rapidly caused to reach a stationary potential. Therefore, even though the sensor has variance, the signal processing circuit connected to the AC coupling circuit can be rapidly and normally operated immediately after the power supply voltage is applied.